The present invention relates to a photovoltaic system with one or more photovoltaic modules producing power that can be supplied to a power mains, and more particularly to a method for operating a photovoltaic system at peak power.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
In conventional photovoltaic systems, the DC current supplied by the photovoltaic modules is converted to an AC current with an electric converter or inverter, and the AC power is then fed to the power grid. Electric converters/inverters are presently commercially available for large systems designed for a total power output of up to 700 kW. However, these systems tend to be expensive. System with larger output power require several electric converters/inverters. For example, at least 9 electric converters/inverters are used today for a solar facility with an output power of 2.5 MW, with each converter/inverter having a power rating of 330 kW.
Although electric converters/inverters have a high power conversion efficiency, the capacitors in the system tend to produce a sluggish response to changes in the operating conditions. For example, the control unit of the converter/inverter may take between 20 seconds and 3 minutes before adapting to changes in the instantaneous incident solar radiation intensity.
The invention is based on the realization that in particular for larger systems, i.e., of 800 kW and above, the converter/inverter should be implemented as a combination of a single DC motor and a single AC generator. DC motors and AC generators with such high power ratings are commercially available. Such a motor/generator combination is much less susceptive to statistic failures due to the smaller number of components than a system using a plurality of electric converters/inverters. The combination also requires less maintenance. It is also advantageous that only a single unit has to be monitored during operation.
Use of a motor-generator combination for operating a solar system is disclosed, for example, in the German utility model application DE 20 2006 002 726 U1. However, this application only addresses the mechanical characteristic of the motor-generator combination and proposes to arrange several pole wheels on a shaft, with each pole wheel operating inside their own stator. This measure is intended to increase the conversion efficiency for regenerative energy generation.
There is still a need for extracting, preferably without losses, the maximum peak power from a photovoltaic system employing a DC motor-AC generator combination connected to a stationary power grid, independent of changes in the ambient temperature and in the incident solar radiation intensity.